The present invention relates to a process and apparatus for measuring the sodium sulfide content of liquid industrial process streams, in which a sample is extracted from a process stream, and reacted with a gas to produce hydrogen sulfide, which is measured as an indication of the sodium sulfide content of the process stream.
Many industrial processes can benefit from the continuous measurement of the sodium sulfide concentration in digester cooking liquors and gas scrubbing solutions. At this time, sodium sulfide, Na.sub.2 S concentrations are determined by transporting a sample of the alkaline solution to a laboratory and analyzing the sample by potentiometric titration or by acidification, purging and trapping the resulting gases. Due to the corrosive nature of the highly alkaline solutions and the high solids content, continuous on-line analysis for sodium sulfide has not been achieved.
In the past, continuous sulfide measurement has been attempted, using specific ion electrodes or oxidation-reduction electrodes. These methods have not proven successful due to the high maintenance necessary to achieve continuous operation. The specific ion electrodes are prone to leakage and contamination and give a variablr output depending upon the temperature and pH of the alkaline solution being tested. The oxidation-reduction electrodes have shown a lack of reliability due to contamination of the electrodes. Oxidation-reduction electrodes also are only useful for the lower Na.sub.2 S concentrations because of the limited output of the detector at the higher Na.sub.2 S concentrations. While the normal range of Na.sub.2 S concentrations to be measured is 40 grams/liter down to 0.01 grams/liter, the metal, oxidation-reduction electrodes are useful only at concentrations of 1 gram/liter or less.
Of the laboratory tests currently used to measure the sodium sulfide content of alkaline solutions, it is felt that the method that acidifies the solution to convert the sodium sulfide to H.sub.2 S and then measure the resulting H.sub.2 S is the most useful approach. This measurement method closely approaches the reactions that occur in combustion processes and should give a measurement of sodium sulfide that will relate more closely to the sulfur gases emitted to the atmosphere.